Even as the OnePlus One is yet to receive the stock Android 5.1 Lollipop update from the handset maker, industrious XDA Developers' senior member, CraZY_BoY, has recently compiled and uploaded the latest unofficial build of CyanogenMod ROM or CyanogenMod 12.1, on the XDA forums.

With due credit to the CM and Euphoria Teams, the latest build of CM12.1 ROM has been bumped to v03.25, which is based on the original CyanogenMod sources.

Interested users may browse for the original CyanogenMod source code via github and also find the necessary OnePlus One kernel source code for CM12 ROM here.

The Android 5.1 CyanogenMod 12.1 ROM can be installed either via TWRP or ClockworkMod (CWM) recovery, using the step-by-step guide posted below.

IBTimes UK reminds readers it will not be held liable for any damage to device during or after the Android 5.1 Lollipop firmware installation. Users are advised to proceed at their own risk.

Pre-requisites

The instructions given in this guide are meant for the European variant of OnePlus One only. Verify device model number by navigating to Settings > About Phone.

Ensure the phone has at least 80% battery charge to prevent unexpected device shutdown (due to battery drain) while installing the ROM.

Back up data and settings using a recommended custom recovery tool, like ClockworkMod (CWM) or TWRP, as it creates a complete image of the existing phone setup. In Recovery, choose Backup and Restore and tap Backup.

Step 4 - Power off the phone and disconnect its USB cable from computer.

Step 5 - Boot the device into Recovery Mode by pressing and holding Volume Down and Power buttons together.

Step 6 - Once the phone enters CWM Recovery, perform a full data wipe task by clicking Wipe Data/Factory Reset. Use the Volumekeys to navigate between options and Power button to select an option in the Recovery Mode.

Step 8 - Perform Wipe Dalvik Cache as well. To do this, go to 'advanced'option and then click Wipe Dalvik Cache. This will help in preventing boot loop issues or any boot ROM related errors during firmware installation.

Step 9 - Then return to the main recovery menu and choose Install Zip from SD card.

Step 10 - Click Choose Zip from SD card. Use volume keys to navigate to the CM12.1 ROM zip file that you copied earlier to your phone's SD card and choose it by tapping the Power button. Confirm the ROM installation on next screen and the installation process will begin.